Optoelectronic devices rely on the optical and electronic properties of materials to either produce or detect electromagnetic radiation electronically or to generate electricity from ambient electromagnetic radiation.
Photosensitive optoelectronic devices convert electromagnetic radiation into an electrical signal or electricity. Solar cells, also called photovoltaic (“PV”) devices, are a type of photosensitive optoelectronic device that is specifically used to generate electrical power. Photoconductor cells are a type of photosensitive optoelectronic device that are used in conjunction with signal detection circuitry which monitors the resistance of the device to detect changes due to absorbed light. Photodetectors, which may receive an applied bias voltage, are a type of photosensitive optoelectronic device that are used in conjunction with current detecting circuits which measures the current generated when the photodetector is exposed to electromagnetic radiation.
These three classes of photosensitive optoelectronic devices may be distinguished according to whether a rectifying junction as defined below is present and also according to whether the device is operated with an external applied voltage, also known as a bias or bias voltage. A photoconductor cell does not have a rectifying junction and is normally operated with a bias. A PV device has at least one rectifying junction and is operated with no bias. A photodetector has at least one rectifying junction and is usually but not always operated with a bias.
As used herein, the term “rectifying” denotes, inter alia, that an interface has an asymmetric conduction characteristic, i.e., the interface supports electronic charge transport preferably in one direction. The term “photoconductive” generally relates to the process in which electromagnetic radiant energy is absorbed and thereby converted to excitation energy of electric charge carriers so that the carriers can conduct (i.e., transport) electric charge in a material. The term “photoconductive material” refers to semiconductor materials which are utilized for their property of absorbing electromagnetic radiation to generate electric charge carriers. When electromagnetic radiation of an appropriate energy is incident upon a photoconductive material, a photon can be absorbed to produce an excited state. There may be intervening layers, unless it is specified that the first layer is “in physical contact with” or “in direct contact with” the second layer.
In the case of photosensitive devices, the rectifying junction is referred to as a photovoltaic heterojunction. To produce internally generated electric fields at the photovoltaic heterojunction which occupy a substantial volume, the usual method is to juxtapose two layers of material with appropriately selected semi-conductive properties, especially with respect to their Fermi levels and energy band edges.
Types of inorganic photovoltaic heterojunctions include a p-n heterojunction formed at an interface of a p-type doped material and an n-type doped material, and a Schottky-barrier heterojunction formed at the interface of an inorganic photoconductive material and a metal.
In inorganic photovoltaic heterojunctions, the materials forming the heterojunction have been denoted as generally being of either n-type or p-type. Here n-type denotes that the majority carrier type is the electron. This could be viewed as a material having many electrons in relatively free energy states. The p-type denotes that the majority carrier type is the hole. Such a material has many holes in relatively free energy states.
One common feature of semiconductors and insulators is a “band gap.” The band gap is the energy difference between the highest energy level filled with electrons and the lowest energy level that is empty. In an inorganic semiconductor or inorganic insulator, this energy difference is the difference between the valence band edge EV (top of the valence band) and the conduction band edge EC (bottom of the conduction band). The band gap of a pure material is devoid of energy states where electrons and holes can exist. The only available carriers for conduction are the electrons and holes which have enough energy to be excited across the band gap. In general, semiconductors have a relatively small band gap in comparison to insulators.
In terms of an energy band model, excitation of a valence band electron into the conduction band creates carriers; that is, electrons are charge carriers when on the conduction-band-side of the band gap, and holes are charge carriers when on the valence-band-side of the band gap.
As used herein, a first energy level is “above,” “greater than,” or “higher than” a second energy level relative to the positions of the levels on an energy band diagram under equilibrium conditions. Energy band diagrams are a workhorse of semiconductor models. As is the convention with inorganic materials, the energy alignment of adjacent doped materials is adjusted to align the Fermi levels (EF) of the respective materials, bending the vacuum level between doped-doped interfaces and doped-intrinsic interfaces.
As is the convention with energy band diagrams, it is energetically favorable for electrons to move to a lower energy level, whereas it is energetically favorable for holes to move to a higher energy level (which is a lower potential energy for a hole, but is higher relative to an energy band diagram). Put more succinctly, electrons fall down whereas holes fall up.
In inorganic semiconductors, there may be a continuum of conduction bands above the conduction band edge (EC) and a continuum of valence bands below the valence band edge (EV).
Carrier mobility is a significant property in inorganic and organic semiconductors. Mobility measures the ease with which a charge carrier can move through a conducting material in response to an electric field. In comparison to semiconductors, insulators generally provide poor carrier mobility.